The present invention relates to the high-pressure shaping of a hollow workpiece. More particularly this invention concerns a system for punching a hole in such a workpiece as it is being formed.
As described in commonly owned U.S. Pat. Nos. 5,628,220 and 5,765,420 of Schafer is it known to bulge-form a workpiece, normally a metal tube, by fitting it to a die and then internally pressurizing the workpiece to such a high pressure that it is deformed outward to take the shape of the tool or die. The result is a very strong cold-formed piece whose external shape perfectly corresponds to that of the die.
It is often necessary to punch a hole in such a workpiece. Rather than do this as a separate step from outside, it is known from German 195 06 067 of Herberg and from U.S. Pat. No. 5,460,026 of Schafer to provide the shaping tool with a punch assembly comprising a guide sleeve whose inner surface is flush with an inner surface of the tool cavity and a punch fitting complementarily in the guide sleeve and movable between an inner position projecting into the tool slightly past the sleeve and an outer position recessed back in the sleeve.
During the initial shaping operation the punch is advanced slightly by an actuator to its inner position to produce an inwardly raised region of the same shape of the punch and having an outer periphery defined by a line of reduced thickness where the material of the workpiece is stretched over the edge of the punch. The punch is then retracted by its actuator and, if necessary, the pressure in the workpiece is increased to drive out the piece and cut it off at the weakened line. This procedure is extremely convenient in that it is done right in the shaping tool during the initial formation operation, and the cut-out piece ends up outside the workpiece.
The problem with this system is that it is relatively difficult to perfectly coordinate the operation of the punch with the process taking place in the tool. The punch is normally retracted when the pressure in the tool reaches a predetermined level which is calculated to correspond to when the workpiece is in solid contact with the interior face of the die cavity. It is, however, more critical that the punch be controlled in accordance with the actual position of the workpiece wall being punched out, but this is not readily ascertainable in the closed tool.
It is therefore an object of the present invention to provide an improved system for punching a hole in a workpiece being shaped by high pressure.
Another object is the provision of such an improved system for punching a hole in a workpiece being shaped by high pressure which overcomes the above-given disadvantages, that is which is quite simple and which ensures that the hole is punched when the workpiece is in the right position.
A high-pressure shaping system has according to the invention a tool defining a cavity and having a bore extending along an axis and opening into the cavity which is adapted to receive a workpiece which can be internally pressurized to expand and engage against inner surfaces of the cavity. A punch extending along the axis has an axially inwardly directed inner end displaceable between an inner position generally flush with the cavity inner surface and an outer position spaced axially outward of the cavity inner surface. A spring unit braced axially inward against the punch and axially outward against the tool has a spring characteristic with a high force/displacement ratio up to a break point and a low force/displacement ratio after the break point for urging the punch inward into the inner position.
Thus when outward pressure exerted against the punch exceeds the break point of the spring, the punch will snap outward and cause a plug of the workpiece to be pushed out into the bore of the tool and separate from the workpiece. The punch is therefore, in effect, actuated by the workpiece so it will only retract when the workpiece is bearing against it with enough force to punch out a plug from its wall by the effect of the internal pressure alone.
According to the invention the spring unit includes a mechanical spring, in particular a belleville or spring washer. Alternately it could be a knee rod, so long as it has the desired spring characteristic is obtained. With pressure plotted on the ordinate and displacement on the abscissa, the spring curve would have an initial very steep part extending almost parallel to the ordinate, then a flat part extending almost parallel to the abscissa, joined at the break point.
The punch in accordance with the invention has an outer end formed with a transversely extending abutment plate bearing on the spring washer. The tool includes a support formed with a chamber holding the abutment plate and spring washer and formed with a passage centered on the axis and through which the punch extends into the chamber. It is possible according to the invention to use an actuator having an inner end bearing on the support for moving the support axially. This actuator allows the punch to remain retracted even after relieving pressure inside the tool so the knocked-out piece stays in the bore until the workpiece has been taken out of the tool. The support has a pair of parts together forming the chamber.